Polio vaccines are tested in our laboratory for potency. Trivalent vaccines are tested for all three polio types (1, 2, and 3). An ELISA procedure developed in our laboratory has recently been updated and standardized. Monoclonal antibodies directed against the D antigen are used as the detecting antibodies in the sandwich. We found two vaccine lots formulated from the same type 2 concentrate to have very different levels of type 2 D antigen in the final container when tested using our standard type 2 monoclonal antibody. This difference was also observed at the final bulk stage, but not at the trivalent concentrate stage. Interestingly, a different type 2 D antigen specific monoclonal antibody indicated not only adequate levels of D antigen in the lot which had barely detectable levels using the first, standard monoclonal antibody, but higher levels than the other lot made from the same type 2 concentrate. In the process of trying to elucidate the cause of this distinct difference in reactivity we reviewed the manufacturing records and the two vaccine lots appear to have been made using identical procedures and components. We tested samples set aside for stability testing covering a period of up to 56 months and samples from a mock production of the vaccine lot in question. The data on stability are difficult to interpret. Although vaccines stored the longest have lower potency than more recently produced vaccines, we do not know the starting potency of these vaccines using the current test as it was not available at the time of their production. The apparent change in potency as measured by the standard monoclonal antibody was not reproduced in the mock production samples. Whether this change in one D epitope is important in terms of vaccine potency and efficacy is not clear. These results indicate that there may be some inconsistency in manufacture which caused the difference between lots. Further examination of other possible causes will be pursued.